Aromatic hydrocarbons, such as benzene, toluene, xylene, etc. are useful as fuels, solvents, and as feeds for various chemical processes. Of the xylenes, para-xylene is particularly useful for manufacturing phthalic acids such as terephthalic acid, which is an intermediate in the manufacture of synthetic fibers such as polyester fibers. Xylenes can be produced from naphtha, e.g., by catalytic reforming, with the reformate product containing a mixture of xylene isomers and ethylbenzene. Separating p-xylene from the mixture generally requires stringent separations, e.g., separations utilizing superfractionation and multistage refrigeration steps. Such separations are characterized by complexity, high energy-usage, and high cost.
Chromatographic separation is an alternative to more stringent separations, such as superfractionation, for removing p-xylene from a mixture of aromatic C8 isomers. Chromatographic separation involves simulating a moving bed of selective adsorbent. Examples of commercial processes in which p-xylene is separated from aromatic C8 isomers by simulated moving-bed separation include PAREX, available from UOP, ELUXYL, available from Axens, and AROMAX, available from Toray. Although a raffinate depleted in p-xylene can be recycled as a feed component to the p-xylene separation step, ethylbenzene will undesirably accumulate in the recycle stream.
In order to overcome this difficulty, p-xylene is conventionally produced in a continuous process (commonly referred to as a xylene loop), in which p-xylene-depleted raffinate is isomerized to reduce the amount of ethylbenzene therein. The isomerization reduces the amount of ethylbenzene in the stream by converting it into an equilibrium or near-equilibrium xylene mixture, e.g., a mixture comprising xylene isomers; diethylbenzene; benzene; and non-aromatics such as C2-C6 olefins and C1-C6 paraffins. One such process involves (a) providing a mixture of aromatic C8 isomers containing p-xylene, (b) separating from the C8 isomers a high-purity p-xylene extract and a p-xylene-depleted raffinate by simulated moving bed adsorption, crystallization, or a combination thereof, (c) catalytically isomerizing the p-xylene-depleted raffinate to produce an isomerate, and (e) recycling the isomerate to step (a).
Vapor-phase isomerization of the raffinate's ethylbenzene is generally needed to achieve an ethylbenzene content of ≦10.0 mole % ethylbenzene per mole of isomerate. However, vapor-phase isomerization has many disadvantages, including high energy consumption, costly and complex process equipment, and high xylenes loss due to conversion of the xylenes in the raffinate into undesirable products such as light gases and heavy aromatics, e.g., by one or more side-reactions such as one or more of cracking, transalkylation, or disproportionation. Attempts to overcome these disadvantages include reducing the quantity of raffinate going to the vapor-phase isomerization, e.g., removing ethylbenzene from the raffinate by (i) superfractionation, as disclosed in French Patent FR-A-2792632, or (ii) using chromatographic ethylbenzene separation in the p-xylene separation stage, as disclosed in U.S. Pat. No. 7,915,471. The separated ethylbenzene is isomerized in a vapor-phase isomerization stage, with the remainder of the raffinate being isomerized in a liquid-phase isomerization stage. The liquid-phase isomerization stage is operated under conditions which lessen undesired cracking, transalkylation, and disproportionation side-reactions. Isomerates from the vapor-phase and liquid-phase isomerization stages are then combined and recycled to stage (a) of the xylene loop.
Even when ethylbenzene is separated for vapor-phase isomerization, with the remainder of the p-xylene-depleted raffinate subjected to liquid-phase isomerization, the vapor-phase isomerization stage contributes to xylene-loop inefficiencies. Some of these inefficiencies result from one or more of (i) the need to vaporize the separated ethylbenzene and then re-condense the vapor-phase isomerate for combining with the isomerate derived from the liquid-phase isomerization stage, (ii) the need to separate unreacted molecular hydrogen vapor for re-use as an isomerization treat gas, and (iii) the need for removing non-aromatics formed during isomerization. Consequently, it is desired to further lessen or even eliminate the need for vapor-phase isomerization.